Effect of coupling agent on natural fibre in natural fibre/polypropylene composites on mechanical and thermal behaviour

To enhance the adhesion between the natural fibre and the thermoplastic matrix, a coupling agent of maleic anhydride grafted polypropylene MAPP is applied. In literature, there are different guidelines of the optimum percentage required of MAPP. Therefore, a systematic work is carried out to optimise the MAPP percent with respect to the type of the natural fibre. Different parameters are investigated namely; Coupling agent ratio to the fibre (0%, 6.67%, 10%, 13.3%, 16.67%), coupling agent source, fibre type (flax, hemp, sisal), and fibre content (30%, 50%). Composite is produced using a kneader and the resulting material is assessed mechanically, thermally, microscopically and for water absorption. For different MAPP source and the natural fibre type, optimum MAPP to fibre ratio is found in average to range between 10% and 13.3% according to the investigated property (stiffness, strength and impact). Increase of MAPP is found to decrease the melting temperature. The thermal behaviour is also linked to the copolymer molecular weight.     

Influence of an Agatha flax fibre location in a stem on its mechanical,chemical and morphological properties

The mechanical properties of flax fibres are analysed as a function of their biochemical and morphological characteristics. The fibres, from the Agatha variety, have been selected from either the top, the middle or the bottom of the stems. The results of each analysis are discussed according to the position of the fibre in the stem and compared among themselves. Considering a flax fibre as a natural composite, this study underlines the complexity of its structure and shows that many parameters intervene in its deformation behaviour.     

Highly aligned flax/polypropylene nonwoven preforms for thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from unidirectional yarns and woven fabrics are known to perform significantly better than composites made from random nonwoven mats, but unidirectional yarns and fabrics are much more expensive to manufacture than random nonwoven mats. Here, we report on highly aligned natural fibre nonwoven mats that can be used as a replacement for unidirectional woven fabrics. A drawing operation is added to the conventional nonwoven process to improve fibre alignment in the nonwoven preforms and the final composites. The modified nonwoven manufacturing process is much simpler and cheaper than the unidirectional woven fabric process because of the elimination of expensive spinning and weaving operations. The composites fabricated from the highly aligned nonwoven mats showed similar mechanical strength as the composites made from unidirectional woven fabrics.     

Polypropylene/natural fibres composites: Analysis of fibre dimensions after compounding and observations of fibre rupture by rheo-optics

Fibre size distribution and the way how fibres are breaking during compounding with polypropylene were analysed. Two types of cellulose-based fibres were used: natural flax and man-made (regenerated) cellulose, Tencel. No significant influence of Tencel initial fibre size on its final size after processing was observed. However, the fibre type and the presence of defects have a stronger effect on the rupture. The value of the average final length of flax fibres was found to be similar to the distance between kink bands.In order to understand fibre rupture mechanisms, they were observed using a rheo-optical system which enabled visualization of fibre break-up. All fibres studied broke by fatigue after an accumulation of strain, contrarily to what is known for glass fibres. Flax fibres broke around the kink bands while Tencel® broke after numerous bending.     

Effect of material and process parameters on the mechanical properties of unidirectional and multidirectional flax/polypropylene composites

Unidirectional (UD) and multidirectional (MD) flax/polypropylene composites were studied. Flax with varying retting degree and boiled flax was used as reinforcement for the UD composites and unmodified and maleic acid anhydride modified polypropylene (MAA-PP) was used as matrix. Multidirectional flax/polypropylene composites were manufactured on laboratory scale and on pilot scale. They were made from needle-punched hybrid flax/PP non-wovens. Normally retted flax as well as boiled flax was used. For the specimens made on pilot scale, a third kind of flax, namely bleached flax was also studied. The influence of different process times and temperatures on the mechanical properties of the composites was analysed. Generally, the composites have adequate good mechanical properties. The unidirectional composites of boiled flax combined with MAA-PP show the best mechanical properties. Contrary to the UD composites, flax treatment did not lead to the expected property improvements for MD composites.     

A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites

Experimental investigations on flax and glass fabrics reinforced epoxy specimens, i.e. FFRE and GFRE, submitted to fatigue tests are presented in this paper. Samples having [0/90]_3S and [±45]_3S stacking sequences, with similar fibre volume fractions have been tested under tension–tension fatigue loading. The specific stress-number of cycles to failure (S–N) curves, show that for the [0/90]_3S specimens, FFRE have lower fatigue endurance than GFRE, but the [±45]_3S FFRE specimens offer better specific fatigue endurance than similar GFRE, in the studied life range (<2 × 10⁶). Overall, the three-stage stiffness degradation is observed in all cases except for [0/90]_3S FFRE specimens, which present a stiffening phenomenon of around 2–3% which could be related to the straightening of the microfibrils.     

NaBF4 as a hygrothermal durability enhancer for glass fibre reinforced polypropylene composites

The long term performance of composite materials is highly desired for their expanding application range. Tuning the interphase properties has been proven to be a practical way to enhance the performance of composites. In this study, short glass fibre (GF) reinforced polypropylenes (PPs) with improved hygrothermal durability were obtained by incorporating NaBF₄ into the sizing and thus the interphases of GF/PP composites. Detailed investigations were performed on the surface properties of sized GFs and the mechanical properties of virgin and aged composites. It was found that the retention in both ultimate tensile strength and Charpy impact toughness of aged composites monotonically increased with increasing NaBF₄ content. The improvement in hygrothermal durability was related to the enhanced fibre/matrix adhesion strength induced by the presence of NaBF₄ as indentified by fracture surface analysis using field-emission scanning electron microscopy and single fibre pull-out test.     

Processing, compatibilization and properties of ternary composites of Mater-Bi with polyolefins and hemp fibres

Ternary composites of a biodegradable thermoplastic matrix, Mater-Bi® (MB), with various polyolefins (PP, HDPE and PS) and hemp fibres (H) were obtained by melt mixing and characterized by SEM, OM, DSC, TGA and tensile tests. The properties of composites were compared with those of MB/polyolefin and MB/H blends. Maleic anhydride functionalized polyolefins were employed as compatibilizers. Crystallization behaviour and morphology of the composites were found to be affected by the composition, phase dispersion and compatibilizer. Thermogravimetric analysis indicated that the thermal stability of the polyolefin phase and fibres was influenced by the composition and phase structure. A significant improvement of tensile modulus and strength was recorded for composites of MB with PE and PS as compared to MB/H composites. The results indicate that incorporation of polyolefins in the biodegradable matrix, compared to binary matrix/fibre system, may have significant advantages in terms of properties, processability and cost.     

Mechanical properties of a self-healing fibre reinforced epoxy composites

Inspired by biological systems in which damage triggers an autonomic healing response, a polymer composite material that can heal itself when cracked has been developed. In this work, compression and tensile properties of a self-healed fibre reinforced epoxy composites were investigated. Microencapsulated epoxy and mercaptan healing agents were incorporated into a glass fibre reinforced epoxy matrix to produce a polymer composite capable of self-healing. The self-repair microcapsules in the epoxy resin would break as a result of microcrack expansion in the matrix, and letting out the strong repair agent to recover the mechanical strength with a relative healing efficiency of up to 140% which is a ratio of healed property value to initial property value or healing efficiency up to 119% if using the healed strength with the damaged strength.     

Effect of corona discharge treatment on mechanical and thermal properties of composites based on miscanthus fibres and polylactic acid or polypropylene matrix

In this study, we investigated the mechanical and thermal properties of composites based on miscanthus fibres and poly lactic acid or polypropylene matrices. The treatment of fibres by corona discharge which results in a surface oxidation and an etching effect as shown by X-ray photoelectron spectroscopy and scanning electron microscopy, leads to an improvement of the interfacial compatibility between matrix and fillers. Hence the homogeneity of materials (checked by X-ray tomography and fractographic evaluation) is better, the mechanical properties measured by classical tensile tests are improved (Young moduli increase around 10-20%). Dynamic mechanical analysis performed on samples leads to similar conclusions (higher modules and slight increase of glass transition temperature hence restricted molecular movement). The thermal stability of composites was investigated by thermogravimetric analysis. While the incorporation of raw fibres leads to a slight decrease of decomposition temperature, it is systematically increased as soon as fillers have been treated.     

Effects of environmental ageing on the static and cyclic bending properties of braided carbon fibre/PEEK bone plates

The use of titanium and steel bone plates to fix fractured limbs can create problems due to stress shielding, bone resorption and subsequent refracture. Here, braided carbon fibre reinforced poly-ether-ether-ketone (CF/PEEK) was evaluated as a possible implant material that could reduce these problems. CF/PEEK bone plates were aged in a simulated body environment for up to 12 weeks and then mechanically tested in 3 and 4-point bending tests. Sample mass increased by around 0.3 wt.%, yet bending stiffness and strength remained unchanged. Scanning Electron Microscopy (SEM) showed no changes in failure modes with age. Braided CF/PEEK shows an excellent resistance to fatigue failure even after prolonged ageing, easily surpassing the fatigue life of commonly used stainless steel alloys such as 316L. In addition, CF/PEEK had half the stiffness of steel for the same static strength, which would reduce stress shielding. Together, the results suggest that CF/PEEK is a highly suitable material for bone plates and should be further investigated for this application.     

Assessing the effect of fibre extraction processes on the strength of flax fibre reinforcement

A number of factors impede the direct translation of fibre properties from plant crop species to natural fibre composites. Commercially available fibre extraction processes introduce defects and degrade the mechanical properties of fibres. This study reports on a novel image based approach for investigating the effect of fibre extraction processes on flax fibre bundle strength. X-ray micro Computed Tomography (μCT) was coupled with uniaxial tensile testing to measure the in-situ fibre bundle cross-section area and tensile strength in flax plant stems. The mean tensile strength result was 50% higher than that of the fibres extracted through the standard commercial process. To minimize fibre damage during fibre extraction, a pre-treatment was proposed via saturating flax plant stems in 35% aqueous ammonia solution. By environmental scanning electron microscopy (ESEM), it was evident that ammonia treatment significantly reduced the extent of damage in flax fibre knots and the optimum treatment parameter was identified.     

Mechanical and thermal properties of chicken feather fiber/PLA green composites

Chicken feather fiber (CFF)/reinforced poly(lactic acid) (PLA) composites were processed using a twin-screw extruder and an injection molder. The tensile moduli of CFF/PLA composites with different CFF content (2, 5, 8 and 10 wt%) were found to be higher than that of pure PLA, and a maximum value of 4.2 GPa (16%↑) was attained with 5 wt% of CFF without causing any substantial weight increment. The morphology, evaluated by scanning electron microscopy (SEM), indicated that an uniform dispersion of CFF in the PLA matrix existed. The mechanical and thermal properties of pure PLA and CFF/PLA composites were compared using dynamic mechanical analysis (DMA), thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). DMA results revealed that the storage modulus of the composites increased with respect to the pure polymer, whereas the mechanical loss factor (tan δ) decreased. The results of TGA experiments indicated that the addition of CFF enhanced the thermal stability of the composites as compared to pure PLA. The outcome obtained from this study is believed to assist the development of environmentally-friendly composites from biodegradable polymers, especially for converting agricultural waste – chicken feather into useful products.     

Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from yarns perform better than composites made from random nonwoven mats. However, the twist structure of conventional ring spun yarns prevents the full utilization of fibre mechanical properties in the final composites. We produced flax/polypropylene commingled wrap yarns in which all flax fibres were twistless. Composites made from the wrap yarn demonstrated significant improvement of flexural modulus. Most currently available low cost natural fibres, such as decorticated hemp, cannot be efficiently made into yarns because of their lack of cohesion. Adding polypropylene fibres to decorticated hemp improved textile processing performance. The polypropylene fibres served as a carrier for the natural fibres during processing and became the polymer matrix in the final composites.     

Mechanical properties of epoxy composites with high contents of nanodiamond

Mechanical properties and thermal conductivity of composites made of nanodiamond with epoxy polymer binder have been studied in a wide range of nanodiamond concentrations (0-25 vol.%). In contrast to composites with a low content of nanodiamond, where only small to moderate improvements in mechanical properties were reported before, the composites with 25 vol.% nanodiamond showed an unprecedented increase in Young’s modulus (up to 470%) and hardness (up to 300%) as compared to neat epoxy. A significant increase in scratch resistance and thermal conductivity of the composites were observed as well. The improved thermal conductivity of the composites with high contents of nanodiamond is explained by direct contacts between single diamond nanoparticles forming an interconnected network held together by a polymer binder.     

Interfacial enhancement of nano-SiO2/polypropylene composites

The authors proposed an approach for manufacturing nano-SiO₂/polypropylene (PP) composites by in situ reactive processing. The key issue lies in that the nanoparticles were covalently bonded to the matrix polymer via polyurethane (PU) elastomer and PP-g-NH₂. Unlike the previous techniques based on graft polymerization, the present one did not need to pretreat the nanoparticles. Taking the advantages of rubber-type grafting polymer (i.e. PU) and interfacial reactive compatibilization with PP-g-NH₂, a synergetic toughening effect was observed for the PP nanocomposites. Only very low concentrations of nano-SiO₂ (1.5–2.5 vol.%) and PU (<4 vol.%) were sufficient to greatly increase notched impact strength of PP. Meanwhile, tensile properties of the nanocomposites were also slightly enhanced.     

Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites

In this work, flexural strength and flexural modulus of chemically treated random short and aligned long hemp fibre reinforced polylactide and unsaturated polyester composites were investigated over a range of fibre content (0-50 wt%). Flexural strength of the composites was found to decrease with increased fibre content; however, flexural modulus increased with increased fibre content. The reason for this decrease in flexural strength was found to be due to fibre defects (i.e. kinks) which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fibre treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fibre/matrix adhesion.     

Study of water behaviour of chemically treated flax fibres-based composites: A way to approach the hydric interface

Silane (Si) and styrene (S) treatments were applied on flax fibres in order to improve their adhesion with a polyester resin and to increase their moisture resistance. The water sorption and permeation kinetics of the composites were correlated with the water sorption behaviour of untreated and treated fibres. An increase of the water barrier effect was observed in treated fibres-based composites in comparison with untreated ones. This was related to the shift-down of water solubility and to a decrease of the water diffusivity in treated fibre-based composites. In the case of (S) treatment, the presence of styrene increased the moisture resistance of the treated fibres and made compatible the fibres and the matrix. In the case of (Si) treatment, a good hydric fibre/matrix interface was obtained due to crosslinking reactions and hydrogen bonding between water molecules and free hydroxyl groups of (Si) treated fibres. In order to interpret water permeation behaviour of composite films, a simple illustrated model is suggested and represented by a schematic view.     

Melt mixed nano composites of PA12 with MWNTs: Influence of MWNT and matrix properties on macrodispersion and electrical properties

Nanocomposites containing four different polyamide 12 (PA12) types and three grades of multiwalled carbon nanotubes (MWNTs) were prepared via small-scale melt processing to study the effect of different MWNTs and the influence of polymer properties on the dispersion of the fillers and the electrical properties of the composites. Under the selected mixing conditions the lowest electrical percolation threshold of 0.7 wt.% was found for Nanocyl™ NC7000 in low viscous PA12. Moreover, big influences of the end group functionality (acid or amine excess) and the melt viscosity of the matrix were found. Composites of PA12 with acid excess showed lower percolation thresholds than those based on amine terminated materials. At constant end group ratio low viscous matrices resulted in lower percolation thresholds than high viscous materials. The best MWNT dispersion was obtained in both high viscous PA12 composites. In these systems the mixing speed was varied indicating an optimum concerning electrical conductivity at 150 rpm as compared to 50 and 250 rpm.     

Interconnected multi-walled carbon nanotubes reinforced polymer-matrix composites

A modified method for interconnecting multi-walled carbon nanotubes (MWCNTs) was put forward. And interconnected MWCNTs by reaction of acyl chloride and amino groups were obtained. Scanning electron microscopy shows that hetero-junctions of MWCNTs with different morphologies were formed. Then specimens of pristine MWCNTs, chemically functionalized MWCNTs and interconnected MWCNTs reinforced epoxy resin composites were fabricated by cast moulding. Tensile properties and fracture surfaces of the specimens were investigated. The results show that, compared with pristine MWCNTs and chemically functionalized MWCNTs, the chemically interconnected MWCNTs improved the fracture strain and therefore the toughness of the composites significantly.